1. Field of the Invention
The present invention relates to an optical disc apparatus and a spherical aberration correction controlling apparatus.
2. Description of the Related Art
Recently, optical discs, such as DVD-ROM, DVD-RAM, DVD−RW, DVD−R, DVD+RW, DVD+R, and the like, have been developed as high density/large capacity recording media.
To date an optical lens having a numerical aperture (NA) of 0.6 has been used as an objective lens of an optical disc apparatus for recording information onto the above-described optical discs and/or reproducing information from those optical discs. In order to obtain an optical disc having a higher density/larger capacity (e.g., a Blue-ray Disc (BD), etc.), the use of a blue-violet laser and an optical lens having an NA of 0.8 or more has been studied.
For the above-described optical discs, a multi-layer structure having two or more recording layers (information surfaces) has been studied.
Hereinafter, lens tilt control and spherical aberration correction performed by conventional optical disc apparatuses will be described (see Japanese Laid-Open Publication No. 2002-157750 (particularly, paragraphs 0070 to 0101, and FIGS. 1 to 8)).
FIG. 13 is a block diagram showing a conventional optical disc apparatus 300. The optical disc apparatus 300 records information onto an optical disc 320 and reproduces information from the optical disc 320.
The optical disc 320 has an information surface for recording information (not shown in FIG. 13) and a protection layer for protecting the information surface provided on the information surface (not shown in FIG. 13). In addition, tracks are provided on the information surface of the optical disc 320.
An optical head section 350 comprises: a light source 303, such as a semiconductor laser or the like, which serves as a light beam generating section; a light receiving section 304 for receiving a light beam which is output from the light source 303 and is thereafter reflected from the optical disc 320; an optical system 352; and an optical drive section 354 for driving the optical system 352.
The optical system 352 comprises an objective lens 301 and a pair of spherical aberration correcting lenses 315.
The spherical aberration correcting lenses 315 are a concave lens 315a and a convex lens 315b. Note that the concave lens 315a and the convex lens 315b may be arranged in a manner different from that shown in FIG. 13.
The optical drive section 354 comprises: a focus actuator 302 for moving the objective lens 301 in relation to the information surface of the optical disc 320; and a spherical aberration correcting actuator 334 for driving the spherical aberration correcting lenses 315 in a manner which changes a distance between the concave lens 315a and the convex lens 315b to prevent the occurrence of spherical aberration.
The optical disc apparatus 300 corrects spherical aberration which is attributed to a predetermined thickness of the protection layer of the optical disc 320 to improve the quality of a signal which is recorded onto or reproduced from the optical disc 320.
Focus control performed by the conventional optical disc apparatus 300 will be described below.
The optical disc apparatus 300 rotates the optical disc 320 (information carrier) at predetermined rpm by means of a disc motor 310.
A light beam output from the light source 303 is focused by the objective lens 301 onto the information surface of the optical disc 320, thereby forming a light beam spot. At the light beam spot, the light beam is reflected from the optical disc 320. The reflected light beam is passed through the objective lens 301 again and is thereafter input to the light receiving section 304.
The light receiving section 304 has four separate regions. Each region generates a photocurrent, the amount of which depends on the amount of light received by the region. The light receiving section 304 outputs the generated photocurrent to a preamplifier 306.
The preamplifier 306 serves as an I/V converter to convert the photocurrent into a voltage. The preamplifier 306 outputs a voltage signal, which has been converted from a current signal, to a focus error signal generator 307.
The focus error signal generator 307 generates a focus error signal indicating a deviation (error) of the light beam spot in a direction perpendicular to the optical disc 320 based on the four signals output from the preamplifier 306. Hereinafter, the focus error signal is also referred to as an FE signal.
For example, the focus error signal generator 307 uses an astigmatism correction method to generate an FE signal. The focus error signal generator 307 outputs the generated FE signal to a focus control circuit 317.
The focus control circuit 317 subjects the FE signal to a filter operation, such as phase compensation, gain compensation, or the like. Thereafter, the focus control circuit 317 outputs the FE signal to a focus actuator driving circuit 309.
The focus actuator driving circuit 309 generates a drive signal based on the FE signal output from the focus control circuit 317, and outputs the generated drive signal to the focus actuator 302.
The focus actuator 302 serves as an objective lens moving section. The focus actuator 302 moves the objective lens 301 in a direction substantially perpendicular to the information surface of the optical disc 320 based on the drive signal output from the focus actuator driving circuit 309, thereby changing the focus position of the light beam.
The focus control circuit 317 controls the focus actuator driving circuit 309 in a manner which causes the FE signal generated by the focus error signal generator 307 to be substantially zero. As a result, the light beam spot is in focus to a predetermined level on the information surface of the optical disc 320. In this manner, focus control is achieved.
Next, the lens tilt control of the conventional optical disc apparatus 300 will be described.
A tilt control section 319 outputs a tilt control signal to a tilt actuator driving circuit 326 in accordance with a command output from a microcomputer 308.
The tilt actuator driving circuit 326 applies offset signals having different polarities to two drive signals output from the focus actuator driving circuit 309 based on the tilt control signal. In other words, the tilt actuator driving circuit 326 makes a predetermined difference between the two drive signals output from the focus actuator driving circuit 309.
With the offset signals from the tilt actuator driving circuit 326, one side of the objective lens 301 is lowered, while the other side thereof is raised. Thus, the objective lens 301 can be tilted depending on the tilt of the optical disc 320, thereby making it possible to cancel a blurry light beam spot, i.e., coma aberration, which is attributed to the tilt of the information surface of the optical disc 320.
In the following descriptions, a technique for controlling the tilt of the objective lens 301 in a manner which cancels coma aberration attributed to the tilt of the optical disc 320, is also referred to as lens tilt control.
Hereinafter, lens tilt control will be described.
FIG. 14 is a schematic diagram for explaining a relationship between a tilt of the optical disc 320 and lens tilt control.
As shown in portion (a) of FIG. 14, the optical disc 320 is ideally placed perpendicularly to a light beam output from the light source 303. When the optical disc 320 is perpendicular to the light beam, the objective lens 301 is also placed perpendicularly to the light beam. Thereby, coma aberration does not occur in a light beam spot on the information surface of the optical disc 320, so that reproduction and recording can be satisfactorily performed.
In actual situations, however, the optical disc 320 may not be perpendicular to a light beam output from the light source 303. For example, when the optical disc 320 is warped, the peripheral portion of the optical disc 320 is not perpendicular, i.e., is tilted, with respect to the light beam output from the light source 303.
It is assumed that the optical disc 320 is tilted from a direction perpendicular to a light beam. In this case, if the objective lens 301 remains perpendicular to the light beam, coma aberration occurs in a light beam spot on the information surface of the optical disc 320, so that the light beam is not correctly brought into focus. As a result, reproduction/recording performance is lowered.
To avoid the above-described problem, the objective lens 301 is tilted to face in the same direction as that of the optical disc 320 as shown in portions (b) and (c) of FIG. 14. As a result, it is possible to cancel coma aberration which occurs in a light beam spot on the information surface of the optical disc 320. Thus, reproduction and recording can be satisfactorily performed.
The objective lens 301 is tilted toward the same direction as that of the tilted optical disc 320. However, the tilt of the objective lens 301 may not be exactly equal to the tilt of the optical disc 320. The tilt of the objective lens 301 may be optimally determined based on an optical constant(s) of the objective lens 301.
Next, spherical aberration correction will be described with reference to the optical disc apparatus 300 of FIG. 13.
A spherical aberration correction control section 335 outputs a spherical aberration correction signal to a spherical aberration correction driving circuit 333 in accordance with a command output from the microcomputer 308.
The spherical aberration correction driving circuit 333 generates a drive signal based on the spherical aberration correction signal, and outputs the generated drive signal to the spherical aberration correcting actuator 334.
The spherical aberration correcting actuator 334 drives the spherical aberration correcting lens 315 based on the drive signal.
The spherical aberration correcting actuator 334 changes a distance between the concave lens 315a and the convex lens 315b (spherical-aberration correcting lenses 315) based on the drive signal output from the spherical aberration correction driving circuit 333 to correct spherical aberration which occurs in a light beam spot. Thus, it is possible to correct a blurry light beam spot (i.e., spherical aberration) which occurs on the information surface of the optical disc 320. That is, spherical aberration correction can be achieved.
Further, spherical aberration correction will be described below.
FIG. 15 is a cross-sectional view showing a state of a light beam in relation to the optical disc 320. As described above, the optical disc 320 comprises a protection layer 321 for protecting an information surface 329, the protection layer 321 being provided on the information surface 329.
Portion (a) of FIG. 15 shows the case where the thickness of the protection layer 321 from the surface of the optical disc 320 to the information surface 329 is optimum, so that spherical aberration does not occur on the information surface 329.
Portion (b) of FIG. 15 shows the case where the thickness of the protection layer 321 from a surface of the optical disc 320 to the information surface 329 is smaller than the optimum value, so that spherical aberration occurs on the information surface 329.
Hereinafter, an inner portion and an outer portion of the light beam will be described separately.
It is assumed that the above-described focus control is being performed. In this case, the protection layer 321 of the optical disc 320 diffracts a light beam output from the light source 303 of the optical disc apparatus 300, so that the outer portion of the light beam is brought into focus B while the inner portion thereof is brought into focus C.
In portion (a) of FIG. 15, spherical aberration does not occur on the information surface 329 of the optical disc 320, so that the focus B of the outer portion of the light beam and the focus C of the inner portion of the light beam share the same position A. In this case, the position A exists on the information surface 329. An equidistant surface from the position A corresponds to the wavefront of the light beam.
However, as shown in portion (b) of FIG. 15, when the thickness of the protection layer 321 is smaller than the optimum value, spherical aberration occurs.
Specifically, the focus B and the focus C are separated from each other and both are displaced from the position A of the information surface 329, i.e., the light beam is defocused. In this case, the wavefront of the light beam does not correspond to the equidistant surface from the position A. In this case, an FE signal is generated irrespective of the outer and inner portions of the light beam, and focus control is performed in a manner which causes the FE signal to be substantially zero. Therefore, the position A indicating the focus position of the whole light beam is located on the information surface 329.
In portion (b) of FIG. 15, solid lines indicate the inner and outer portions of a light beam when spherical aberration occurs, while broken lines indicate the inner and outer portions of a light beam when spherical aberration does not occur.
Also, when the thickness of the protection layer 321 ranging from a surface of the optical disc 320 to the information surface 329 is greater than the optimum value, the focus B and the focus C are separated from each other and both are displaced from the position A of the information surface 329, i.e., the light beam is defocused, in a manner similar to that shown in portion (b) of FIG. 15.
Thus, particularly when an optical lens having a large numerical aperture (NA) of 0.8 or more is used, spherical aberration, which occurs due to the uneven thickness of a protection layer for protecting the information surface of an optical disc and errors in the distance between the surface of a multi-layer optical disc and the information surface of each layer, is not negligible. In order to remove the influence of such spherical aberration, a method for switching the levels of correction of spherical aberration so as to achieve a multi-layer structure has been proposed.
In the optical disc apparatus 300, when spherical aberration occurs, spherical aberration is corrected by changing the distance between the concave lens 315a and the convex lens 315b to modify the outer and inner portions of a light beam.
According to the above-described conventional technique, an objective lens is tilted so that coma aberration which occurs in a light beam spot due to the warp of an optical disc can be canceled.
However, in the conventional technique, when the objective lens is tilted so as to cancel coma aberration, spherical aberration occurs due to the tilt of the objective lens in addition to that which occurs due to influences, such as the uneven thickness of the protection layer of a disc or the like.
Therefore, even if coma aberration is canceled by the lens tilt control, additional spherical aberration is caused by the lens tilt control. As a result, recording/reproduction performance is lowered.